In the prior art, when effecting modifications to cylinders in a dryer group in a paper machine, e.g., machining or drilling drying cylinders, the drying cylinders were removed from their place and transferred away to be machined separately. Thus, in the prior art, it is known to convert ordinary non-perforated cylinders in a dryer group into so-called "Uno Vac" cylinders by making holes in the cylinder mantle. In the current assignee's Finnish Patent No. 91,135 (corresponding to U.S. Pat. No. 5,090,846, incorporated by reference herein), a method and a device are described by whose means the perforation of the cylinder mantle can be carried out on the site in the paper mill without removing or detaching the cylinder. In this arrangement, a drilling machine moves on support of separate guides in the axial direction of the roll. During the machining process, the drilling unit is supported both on the mantle face of the cylinder to be machined and on the mantle face of an adjacent cylinder. The drilling unit includes a spindle box of a multi-spindle drilling machine which comprises a unit for rotation of the drill bits and connected power transmission means for transferring the rotation to all of the adjacent drill bits at the same time. The drilling unit further comprises a cylinder by whose means the jig of the drill bits, i.e., the drill guide, is displaced or guided into contact with the mantle face of the cylinder to be machined. In this case, since the equipment rests both against the cylinder to be machined and against this adjacent second cylinder, which constitutes a support base for the guides, a continuous, robust construction is formed during machining, which permits a drilling free of disturbance. The drilling unit further comprises a spindle box displaceable with respect to the main frame and in this box, a separate motor for displacing the spindle box with its drills. Further, the drilling unit includes a motor for rotating the drill bits. The drill bits are arranged to drill holes in a horizontal direction.
In the construction in accordance with Finnish Patent No. 91,135, the drill bits are placed in a fixed position in relation to one another. In such a case, the spacing of the grooves on the roll must also be fixed, because it is preferable that the perforations are placed expressly in the bottoms of the grooves. However, the spacing of grooves can vary from cylinder to cylinder.
With further respect to the prior art, reference is also made to the current assignee's Finnish Patent No. 91,834 (corresponding to U.S. Pat. No. 5,334,259, in which an arrangement is described for improving the method and the equipment in accordance with the Finnish patent mentioned above. In this improved arrangement, the spacing of adjacent drill bits in the drill equipment can be adjusted in a desired way.
A drawback of the prior art constructions is that the installation of the guides necessary for fastening of the drill guide has been time-consuming, and in some cases, there has not been enough space to fasten the guides to the cylinder adjacent to the cylinder to be drilled, e.g., if a machine construction with dense spacing of cylinders and a narrow gap between cylinders is involved.
It is a further drawback of the prior art constructions that, in some cases, it has not been possible to carry out the drilling of the holes by means of the drill guide immediately from the edge of the cylinder, but the holes to be placed in the edge areas have had to be drilled manually.
It may be considered a further drawback in the prior art constructions that, when drilling takes place from the side (in a horizontal direction as in both Finnish patents referenced above) or from below (vertically upward), chips etc. may enter into the drill guide and cause problems and also, their removal is problematic.